1. Field of the Invention
This invention relates to a toner for developing electrostatic images. More particularly, this invention also relates to a toner for developing electrostatic images which comprises a binder resin incorporated with a colorant and a charge control resin.
2. Related Background Art
Toners used to develop electrostatic latent images are required to have a proper charge quantity in order to obtain sharp images with less fog. The toners are further required to undergo less changes with time in charge quantity, and do not cause any changes such as serious attenuation of charge quantity and agglomeration because of environmental changes (e.g., changes in temperature and humidity). The reason therefor is as follows:
That is, with a decrease in charge quantity because of its attenuation from the value set originally, the toner may more come to scatter, and hence there is a possibility of causing problems such that the fog occurs to blacken background areas which should normally be left white and that the developing unit is contaminated at its surrounds by the toner.
In order to achieve proper charge quantity, usually, charge control agents are added to toners. Nigrosine dyes, metal-containing azo dyes and so forth have hitherto been in wide use as charge control agents for providing the toners with positive electric charges; and compounds of metals containing a heavy metal, as charge control agents for providing the toners with negative electric charges. However, as regards these charge control agents, it is pointed out that they must further be improved in environmental properties and safety.
Meanwhile, a technique is also known in which a charge control resin is added to the toner (Japanese Patent Application Laid-open No. H02-167565). However, a phenomenon may take place in which a negative charge control resin coming present at toner particle surfaces adheres to charge-providing members (a phenomenon called “toner-spent”). Hence, there is a possibility that the charge-providing members come to have a low charge-providing ability as a result of long-term service.